The unusual combination of anisotropic and fluid properties in liquid crystals have resulted in their use in a multiplicity of electro-optical switching and indicating devices. In these, their electrical, magnetic, elastic or thermal properties can be used for the purpose of orientation changes. Optical effects can then be achieved by means of their birefringence ("birefringence mode"), the intercalation of dichroically absorbing dyestuff molecules ("guest-host mode") or light scattering.
In this connection, the nematic phase (N) and the smectic high-temperature phases S.sub.A and S.sub.C or their chiral versions N*, S.sub.A * and S.sub.C * have, apart from a few exceptions, hitherto been used.
The S.sub.A and S.sub.C phases have a layer structure with randomly distributed molecular centers of mass inside a layer. They are distinguished by the fact that in the S.sub.A phase the director n is perpendicular to the plane of the layer, i.e. parallel to the layer normal z (definition of the orthogonal phases), but in the S.sub.C phase a tilt is present which is specified by the angle .theta. between n and z (definition of the "tilted phases").
In the "bookshelf" geometry proposed by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980)), the ferroelectricity of the S.sub.C * phase can be exploited for the purpose of an electro-optical effect. This is based on the existence of two stable states between which a rapid switching takes place typically in 50 .mu.s in an electric field of 10.sup.7 V/m (R. B. Meyer, L. Liebert, L. Strzlecki and P. Keller, I. Phys. (Paris) Letters 36, L-69 (1975)).
This ferroelectric effect is notable for an extremely non-linear electro-optical characteristic curve.
It is known that in the S.sub.A * phase (chiral S.sub.A phase), a related, but linear process occurs which, following Garoff and Meyer (S. Garoff and R. B. Meyer, Phys. Rev. Lett. 38, 848 (1977)), is called the electroclinic effect. It is based on a field-induced angle of tilt .theta. in the per se orthogonal S.sub.A * phase, which is proportional to the electric field E extending parallel to the smectic layers. The magnitude of the electroclinic effect is specified by the differential coefficient (d.theta./dE). In the more highly ordered phases, for example S.sub.B, S.sub.E etc., the molecular centers of mass are not randomly distributed inside a layer but are arranged regularly, like in a crystal lattice. This higher order is matched by a higher viscosity and solidity of the layers. The more highly ordered smectic phases, in particular the orthogonal S.sub.B and S.sub.E phases, were not therefore considered for use in electro-optical components in which rapid responses of the liquid-crystalline layers to changes in an applied electric field are required.
Surprisingly, it has now been found that S.sub.B * (chiral S.sub.B) phases and S.sub.E * (chiral S.sub.E) phases exhibit an electroclinic effect with unexpectedly short response times down to 1.5 .mu.s, the electroclinic coefficient d.theta./dE being, in addition, larger than in the S.sub.A * phase.